Korg late MS20 filter by René Schmitz ,follow the link http://www.uni-bonn.de/~uzs159/index.html for more circuit ideas by René Schmitz along with nice tutorials on OTA's ,temco and temperature compensation .

Diode ladder VCF by Yves Usson ,as part of the Yusynth Modular , page dedicated to this filter is here http://yusynth.net/Modular/index_en.html with details on construction ,calibration , wiring and demo. From the notes on the page :

Well, here is another classical and very special filter based on the EMS diode ladder filter!
This filter has a sound of its own and is the signature of the classical Pink Floyd's song "On the run".

My aim here was to design a quite faithful copy of this mythic filter while using components that minimize the burden of selecting a great number of matched transistors. This is why I used transistor arrays (CA3046, CA3096). However, because the CA3096 has become a rare part and is now overpriced, I have also designed a PCB with discrete trannies to replace it. As for all the other filters of my YUSYNTH project, I have added an input and an output buffer to adapt this filter to the input and output levels of my modular system.
I also added an input for voltage control of the resonance : everything was there in the original module but was not made available, probably because of the limited size of the patch pin matrix of the AKS/VCS3...
Because, my power supply standard is +15/-15V, I have included a power regulator IC (78L12) to convert +15V to +12V and a single transitor voltage regulator -15V to -9V (I wanted to use a 79L09 but changed my mind when I realized that this voltage regulator is quite a rare part and is difficult to source).

The RESONANT EQUALIZER (EQ) is a unique ten-band filter designed specifically for electronic sound synthesis and processing. Except for the top and bottom frequency bands, all other bands are spaced at an interval of a major seventh. This non-standard spacing avoids the very common effect of an accentuated resonance in one key, as will be the effect from graphic equalizers with octave or third-octave spacing between bands. Spacing by octaves will reinforce a regular overtone structure for one musical key, thereby producing regularly spaced formants accenting a particular tonality. The Resonant Equalizer's band spacing are much more interesting, producing formant peaks and valleys that are similar to those in acoustic instument sounds.

There are three equalized outputs, two which mix the alternate filter bands, and one which is a mix of all filter bands. The upper (up arrow COMB) lets pass the outputs of frequency bands at 61 Hz, 218 Hz, 777 Hz, 2.8 kHz, and 11 kHz. The lower (down arrow COMB) mixes the other bands (29 Hz, 115 Hz, 411 Hz, 1.5 kHz, 5.2 kHz). This equalizer is different from other equalizers in that the bands can be set to be resonant. When the knobs are in the middle position, the response at the main EQ Output is flat. When the knobs are positioned between the 9 and 3 o'clock position, up to 12 db of boost or cut is set at the band. If the knob is set beyond the 3 o'clock position, the band will become resonant, simulating the natural resonance of acoustic instrument formant structures. Below the 9 o'clock position, increased band rejection is achieved.

T914 Extended Fixed Filter Bank by Carsten Toensmann ,for full description and schematics as well as demo of this Passive Moog Type Filter Bank visit : http://www.analog-monster.de/mmt914_en.html description taken from the page :

And yes - another Moog filter is cloned, the 914 Extended Fixed Filter Bank. Other synthesizer concepts call them Formant Filter, but anyway - these filters don't have any voltage control but attenuate or emphasize the signal content concerning a certain frequency band by manual control (potentiometer).

The T914 consists like the original 914 of 14 filter bands. There are a low pass filter, 12 band pass filters and a high pass filter. The center frequencies of the band pass filters are:

The behviour of the module is non-linear. If you process a square wave signal and enable all filter bands the output is no square but something complex with peaks and notches on the scope and it sounds much more "vivid" than the input does.

recently im abusing of this incredible filter designed by a friend of mine
is really interesting for his interaction between resonance and frequency
and the sound has his own character
can run on different voltages from 9v to 12v,
at 12v can really scream!!!
no cv controll but in combination with the sn voice module could fit without problems behind the same panel for example

"There seem to be two VCF circuits in the schematics (you'll have to rotate them clockwise by 90 degrees in your PDF viewer for them to make sense), one in the "Poli" section (upper right corner of page 6) and one in the "Mono" section (upper right corner of page . They're quite strange. It looks like they're using JFETs as variable resistors; this is usually easy to do in a predicable way if one side of the "resistor" is grounded, but here they're "floating," which is likely to induce all sorts of odd behavior. The Poli one seems to be an cascade of two RC stages, but without buffering between them, so the poles of the filter will be spread out a bit, and I don't seem to see any feedback. The Mono version seems to have a feedback loop from the output back to the input, but that feedback loop itself has some unbuffered RC stages in it, and there's a swtich that you can use to switch this feedback loop in and out. I'd love for someone to rig this up and hear how it sounds! (This would probably make a good one person project.) "

recently im abusing of this incredible filter designed by a friend of mine
is really interesting for his interaction between resonance and frequency
and the sound has his own character
can run on different voltages from 9v to 12v,
at 12v can really scream!!!
no cv controll but in combination with the sn voice module could fit without problems behind the same panel for example

mate, brilliant, i was after a really simple single sided LPF to put after hex inverters battery acid distortion, this will be perfect.

This is a simple sallen-key VCF using a miniature tube (1J17B) as the main amplifier. There are a few opamps in the signal path, but they are only used with a gain of 1 as inverting and non-inverting buffers. The circuit is based on Ken Stone’s tube VCA which uses the 1j24B. The cutoff frequency is controlled using a dual vactrol.

The core amplifier which is based around the tube is somewhat different than in Ken Stone’s VCA. I have removed the voltage control of the gain, and changed the tube’s biasing. The reason for this is that I noticed the tube going into cutoff mode with large signal inputs. That problem was corrected using an input resistor with greater impedance. The control grid resistor, screen grid, and plate resistors were adjusted so that the distortion is more or less symmetrical. A fixed gain of about 3 is used for the tube amplifier. Resonance is reduced rather than increased as the signal is attenuated in the feedback path. With this setup, the tube starts to clip on signals greater than about +-2V, but the distortion is what I was after. I think it sounds nice with signals at or above +-5V. The maximum signal output is around +-6V.

At high resonance levels, the filter will oscillate but requires some input to get it started. After oscillating starts, the signal can be removed. While oscillating, a large input is required to overtake the resonant frequency. Several different sounds can be achieved by adjusting the input level while at maximum resonance.

This module was inspired by Richard Brewster's Mixer-Comparator module. I decided I wanted additional functionality and added a Full Wave Rectifier as an alternate output. A jumper strap on the PCB selects either AC or DC coupling for the full wave rectifier. For AC coupling the gain is x2 so the output is a full +/- 5 volts.

This circuit board was designed to allow for easier assembly of 7 pin tube based synthesizer circuits. Going on the number of requests I have had for a tube module that runs on +/- 15 volts, the first project I present using this board is exactly that. It is a simple VCA that doubles as a wave folder/distortion unit. There are no dangers in connecting this to solid state modules due to the voltages in use, and the fact that this is really a voltage controlled attenuator, and not an amplifier. There is about 50% signal level loss.

Serves to mix, compress and invert control voltages. Each channel has two control voltage inputs and an internal voltage source. Particularly useful for obtaining fine pitch control, transposition capability, and range compression of control voltage sources.

The Voltage Controlled Quadrature Function Generator Project by Thomas Henry ,page dedicated to this module is at Birth of a Synth by Scott Stites http://www.birthofasynth.com/Thomas_Henry/Pages/QFG.html visit for details on construction , calibration procedure and more .From there the brief description :

Through the years, Thomas Henry has designed a lot of synthesizer circuits - you name it, he's designed at least one version of it - VCOs, envelope generators, VCAs, filters, sequencers, flangers, phase shifters, drum voices...pretty much the gamut of things that make our synthesizers tick. All of them have his indelible mark of a carefully considered level of functionality coupled with a high standard for the elegance of the design - in other words, Thomas hates to open cans of beans with dynamite.

All of these circuits are worthy of your synthesizer, so it's hard to find any one or more of those circuits to differentiate by calling it truly "classic" over the others, but if I had to, there are certainly a few that I would choose. One of them would be his Supercontroller circuit; another, in my opinion, would be the relatively new SN-Voice. But, surely included in this group would be his Quadrature Function Generator. What makes it a classic to me is the approach Thomas took, which was, well, ninety degrees out of phase from any other approach to generating quadrature waveforms.

My latest VCA is the one in the picture VCA3. This is the VCA that I use inside my so called "Blue Box". It is based on VCA 2, but shows a number of minor improvements (which can be retrofitted to VCA2). I have also shown my external pot connections. These were left out on the VCA2 drawing. I added a trimpot at the inverting input, so that the output offset can be eliminated. This makes it DC accurate. The adjustment is somewhat tricky, since it interacts with the CV rejection pot. You will have to adjust the trimmers alternatingly. Its well worth the effort, with careful adjustments you get it basically thump free. (But beware that your breath might have an impact on the setting, best is to insulate the pair with styrofoam or something.) Also there is the trimmer called "deaf-zone elimination" by which you can shift the point where the gain goes to 0, so that with 0V input (and the manual pot at the lowest setting) the VCA is just closed

Serge VCA as suggested by Serge Tchernepnin in an old issue of Electronotes . Schematic shared by Jörgen Bergfors ,visit http://hem.bredband.net/bersyn/VCA/vca_shootout.htm Jörgen's DIY VCA shootout for in detailed analysis on various VCA's . On this LM13700 based circuit Jörgen wrote :

It has the resistors on the inputs connected to -15V instead of ground. The theory was that the linearizing diodes would be more effective that way. The output stage is like LM13600 VCA 1. The contol caracteristics are the same as the other LM13600 VCAs. Whatever the theory behind ths circuit was, it seems to be wrong. This circuit performs terrible, compared to the one from the National Semiconductor application notes. The distortion is very high regardless of signal level. It seems the linearizing diodes overcompensate the linearity error in the amplifier. Another problem is that the signal bleedthrough is alarmingly high. Only 59 dB attenuation at 10 kHz is not enough.

Audio signals are fed into inputs one, two and/or three, and levels of relative mixing are set with the three input pots.

The overall gain can be controlled by adjusting the pot labeled GAIN and or by feeding control voltages into the V.C. input jacks.

For use in conjunction with the envelope generator, use the following steps.

Plug the audio signal (from one of the oscillators) into one of the VCA Mixer inputs and turn up the pot for that channel about half way. Feed the out jack into an audio amplifier, guitar amplifier, or whatever sound system is being used.
Turn up the master GAIN pot until a signal is heard.
Turn the envelope generator output level pot for Env #1 all the way down.
Turn the GAIN pot on the VCA Mixer counterclockwise until the audio signal just barely disappears.
Turn the output level pot on the Env Gen #1 to about 12 o'clock.
Now a signal should be heard in accordance with the setting of Env. Gen. #1 whenever a KBD trigger or other trigger is fed into the trigger jack of the envelope generator.

Envelope generators #2 and #3 can be used to control other modules while #1 is controlling the VCA Mixer. For example, to sweep the V.C. Filter and/or Ocs. Frequency.

On final note: Overdriving the VCA Mixer overrides control voltage rejection. You should keep the control voltage pot at about the 12 o'clock position, generally. Lack of control voltage rejection makes a "popping" sound.

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